This invention relates generally to rider propelled vehicles and more particularly to a drive assembly for rider propelled vehicles.
Rider propelled vehicles (e.g., bicycles, tricycles and the like) are universally driven drive assemblies having a pair of cranks rotated about an axis by pedalling action of the rider's legs. Pedals may be mounted directly on the cranks, or indirectly connected to the cranks by of connector rods, such as when reciprocating pedalling action is employed. Typically, the cranks of the drive assembly are fixed on opposite ends of a shaft and permanently located 180.degree. apart. A well known drawback to the typical configuration is that the cranks will simultaneously occupy top and bottom "dead center positions" in which only a very low torque is applied to the crank shaft. More specifically, at the beginning of the power stroke of each of the rider's legs (that is, when the leg begins to be extended by the rider from a flexed position), force applied by the rider's leg to the crank tends to be directed through or very near the axis of rotation of the crank shaft. The other leg provides no assistance because it is fully or nearly fully extended and any force applied by that leg would also be directed along a line at or near the axis of rotation. As a result, a very low torque is generated by the drive assembly.
Low torque application is particularly troublesome when first initiating motion of the vehicle and when travelling up steep grades at low speeds. The rider may attempt to overcome this by pushing the vehicle until the cranks move out of their dead center positions, or by changing his body's position on the bicycle so that the force applied by his legs is directed along a line spaced further from the axis of rotation. Either of these solutions is inconvenient for any rider and nearly impossible for some riders who have sufficient leg strength to propel the bicycle but through age or handicap find it difficult to change the position of their bodies. In part to facilitate changing body position, bicycle seats have been located relatively high and almost directly above the pedals. Because of the location of the seat relative to the pedals and need to change body position, back rests are not employed. The absence of a reaction surface, such as could be provided by a back rest, limits the force the rider can apply to the cranks.
The problem of the top and bottom dead center positions is acute when reciprocating motion is employed to power the drive assembly. In that event, the cranks are connected by rods to pedals driven in reciprocating, rather than rotary fashion. The rods fix the direction by which forces may be applied to the cranks. Thus, the top dead center position cannot be overcome by the rider changing his body position.
Prior solutions to this problem are disclosed in my U.S. Pat. Nos. 3,779,099, 3,906,807, 4,029,334 and 4,159,652. Generally, the solution disclosed in these patents is to provide two cranks on each side of the vehicle, a drive crank fixedly connected to the sprocket and a pedal crank connected to the pedals (via connector rods in the case of reciprocating motion). These cranks are interconnected by a torque transmission assembly which causes the pedal crank to be accelerated and decelerated relative to the drive crank during each orbit of the drive crank about its axis of rotation. The pedal cranks on each side of the vehicle are also accelerating and decelerating with respect to each other such that they are no longer constantly maintained at 180.degree. separation. Thus, when one of the pedal cranks is at its bottom dead center position the other pedal crank is separated by less than 180.degree. (measured in the direction of rotation of the pedal cranks) so that the other pedal crank does not simultaneously occupy its top dead center position. Therefore, the rider is able to apply a substantial torque to the drive assembly throughout each rotation of the pedal cranks without changing his body position.
My prior inventions operate in a satisfactory manner to eliminate the problem of the top (and bottom) dead center position in the drive assemblies of rider propelled vehicles. However, I have found it desirable to improve upon certain aspects of these inventions. In each of my prior inventions, the torque transmission assembly is exposed to the environment which could result in the various cams and bearings of the assembly becoming fouled with dirt, grease and other debris, or could subject the assembly to damage by impact. Some of my prior inventions (e.g., those disclosed in U.S. Pat. Nos. 4,029,334 and 4,159,652) employ cam followers moving in a cam raceway. The sliding action of the cam follower in the cam raceway may result in wearing of the follower and raceway unless more expensive materials are used.
The exposed disposition of the torque transmission assembly causes a bicycle to have an unusual appearance which may frighten off the ordinary bicycle rider for whom the inventions may provide the greatest advantage. Further in some instances (e.g., U.S. Pat. No. 4,159,652), the result is achieved by separating the axis of rotation of the pedal cranks from that of the drive cranks, which also gives the drive assembly a nonstandard appearance.